1. Field of the Invention
The present invention discloses an improved process for oxidizing alkyl aromatic hydrocarbons and/or their partially oxidized intermediates to produce aromatic carboxylic acids. The process involves liquid-phase oxidation in the presence of a catalyst of cobalt-manganese-bromide and one or more than one transition metal or lanthanide metal component, in an aliphatic carboxylic acid having 1.about.6 carbon atoms such as acetic acid. In particular, one or more than one type of transition metal or lanthanide metal components are added to the conventional catalyst system of cobalt-manganese-bromide during the oxidation with oxygen-enriched gas.
The decrease in rate of the oxidation reaction of an alkyl aromatic substrate with oxygen-enriched gas in the later half of the reaction, and the precipitation of a catalyst therein were significantly deterred in the present process as compared to the conventional processes with the catalyst system of cobalt-manganese-bromide. The quality such as chromaticity of a carboxylic acid product was also significantly improved, and the side reactions to carbon dioxide were decreased in the process.
2. Description of the Related Art
As disclosed below, methods of manufacturing aromatic carboxylic acids are well known and widely used commercially. For example, a method of manufacturing of aromatic carboxylic acids such as terephthalic acid (TPA), isophthalic acid (IPA), phthalic acid, phthalic anhydride, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydride, trimesic acid, pyromellitic dianhydride, 4,4'-biphenyldicarboxylic acid, and benzoic acid by oxidizing alkylaromatic compounds or the oxidized intermediates thereof, in the presence of cobalt-manganese-bromide, from such alkylaromatic compounds as para-xylene, para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), meta-xylene, meta-tolualdehyde, meta-toluic acid, 3-carboxybenzaldehyde, ortho-xylene, dimethylnaphthalene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), pentamethylbenzene, hexamethylbenzene, 4,4'-dimethylbiphenyl and toluene is well known(for examples, U.S. Pat. Nos. 2,833,816 and 5,183,933). Such aromatic carboxylic acids are used as raw materials for manufacturing polyester after appropriate purification such as hydrogenation, etc. (U.S. Pat. No. 3,584,039). Also, polyester is widely used as a synthetic fiber, film, etc.
As an oxidant during the manufacturing of aromatic carboxylic acids, air, oxygen, or oxygen diluted with inert gas can be used. In a commercial plant, air is mainly used as an oxidant. When an oxygen-rich gas is used as an oxidant during the production of aromatic carboxylic acids, there the advantages lie in high efficiency of the gas and raw materials and the decrease in by-products and wastes. Because of these reasons, a number of cases have been known to use pure oxygen or oxygen-enriched gas as a reactant gas therein. In other words, some methods to produce TPA with pure oxygen or oxygen-enriched gas are known. For example, TPA can be produced by oxidation of para-xylene with oxygen-enriched gas and recycling of the discharged gas in the reactor into the liquid phase of the oxidation reactor (U.S. Pat. No. 5,596,129). It is also disclosed that TPA could be obtained with decreased losses of reactants and solvents by means of enhancing the efficiency of mixing and facilitating the contacts between the reactants and the reaction gas (U.S. Pat. Nos. 5,696,285, 5,523,474, and 5,371,283). It is taught that the problematic hazards in the oxidation of para-xylene such as explosion and fire can be prevented by recycling of the carbon dioxide-enriched gas produced in the oxidation (U.S. Pat. No. 5,693,856).
Nevertheless, the production methods of aromatic carboxylic acids with oxygen-enriched gas have not been applied commercially as of yet because there may be some problems such as safety problems and precipitation of metal catalysts such as manganese (Mn.sup.2+ -&gt;Mn.sup.3+ -&gt;MnO.sub.2). The precipitation of a catalyst may cause reduction in activity, deterioration of quality such as chromaticity of the product, and decrease in efficiency of the catalyst system due to deviation from the optimum catalyst composition.
As for the catalyst system in the production of aromatic carboxylic acid, a transition metal-bromide catalyst system such as cobalt-manganese-bromide is usually used and some transition or lanthanide metal components, such as Ce, Zr, Hf, Fe, Cr, Mo, etc., can be added to improve the reactivity and quality and to decrease the side reactions. Cerium can be employed to decrease the Br concentration (U.S. Pat. No. 5,453,538). Zirconium, hafnium, nickel, iron, molybdenum, and chromium can be used to improve the efficiency of the catalyst system (U.S. Pat. Nos. 5,359,133 and 5,112,992).
Nevertheless, the case is not known in which one or more than one transition or lanthanide metal components are added to prevent a decrease in reactivity and the precipitation of a catalyst such as manganese during the oxidation of aromatic substrates by oxygen-enriched gas.
Therefore, it is very important to develop a production method of aromatic carboxylic acids with oxygen-enriched gas without the decrease in reactivity and precipitation of catalyst components.